The requirement for a color cathode-ray tube data display imposes high standards on the operating performance of the tube and particularly on the size and shape of the spot traced on the screen by the cathode-ray beam.
One method used in measuring the spot size of a monochrome cathode-ray tube is the so-called "shrinking raster" technique. A raster of lines is traced on the screen and the space between the lines reduced until the screen appears to have uniform brightness. If the distribution of light output at the spot is gaussian, this technique gives the line width at about 60% of peak brightness. If, however, the distribution is not gaussian, the determination of line width is likely to vary considerably from the 60% value. The "shrinking raster" technique is likely to be in error by a factor of two as against the perceived line width and is not effective for accurate automatic quality control.
Apart from the unsatisfactory nature of the "shrinking raster" technique, the presence of the shadow mask in a color cathode-ray tube presents further difficulties to the observer. The electron beam passes through several of the apertures in the shadow mask and cannot directly be perceived. The shrinking raster technique gives the line width value in the range 50 to 85% of peak intensity. The most usual techniques for determining spot size in a shadow mask cathode-ray tube involve either comparing the spot on the screen with a standard aperture film--the so-called spot gauge--or by counting the number of illuminated phosphor dots, both of which techniques are highly subjective and are not capable of accurately describing a non-circular spot.